Lithography (also known as photolithography) is a commonly used technique in the integrated circuit manufacturing process. In a lithography process, a material layer that is to be patterned is formed first. A photo resist is applied on the material layer. The photo resist is exposed to light through a lithography mask, which includes transparent patterns that allow the light to pass through, and opaque patterns that block the light. The exposed photo resist is developed to form a patterned photo resist. In the patterned photo resist, depending on whether the photo resist is negative or positive, either the portions that were exposed to the light are removed, or the portions that were not exposed to the light are removed. The material is then etched, wherein the portions of the material layer protected by the remaining portions of the photo resist remain, and the un-protected portions of the material layer are removed.
The lithography mask is made prior to the lithography process. The lithography mask is formed by forming an opaque layer over a transparent layer, and then patterning the opaque layer. The regions where the opaque layer is left un-removed are the opaque portions of the resulting lithography mask, and the regions where the opaque layer is removed are the transparent portions of the resulting lithography mask.
In the patterning of the lithography mask, however, defects may occur. For example, some portions of the opaque layer intended to be removed may actually be left un-removed. These portions need to be removed in a repair process. In the repair process, an electron beam or an ion beam may be used to scan and remove the defective portions.